This invention relates to an electromagnetic-shielding gasket placed around a joint between conductive housings of electronic components or between the conductive housing and a cover member.
These days, office equipment such as electronic typewriters and printers, household appliances such as washing machines and vacuum cleaners, and the like increasingly use electronic components like microcomputers. Even automobiles equipped with electronic control systems for fuel injection are increasing rapidly. Thus, the number of microcomputers is increasing. Moreover, the clock frequency of microcomputers is also increasing. Electronic components are generating increasing amounts of electromagnetic noise. This noise leaks through gaps in the housing of electronic components, and may be transmitted through signal conductors to other electronic components, thus causing malfunctions or damaging the other electronic components.
In the related art, gaps in the housing of the components are filled with an electromagnetic-shielding gasket consisting of conductive metal (referred to as "the metallic gasket"), an electromagnetic-shielding gasket consisting of elastomer containing conductive-metallic particles or carbon black (referred to as "the elastomer gasket"), or an electromagnetic-shielding gasket consisting of elastomer coated with meshed metal (referred to as "the meshed gasket") to prevent electromagnetic waves from entering the electronic components.
By filling the gap in the housing with the metallic gasket, the elastomer gasket, or the meshed gasket, the housing conducts electricity to the cover or another housing. Electromagnetic noise generated by the electronic components in the housing or entering from outside into the housing is reflected and absorbed by the conductive housing and the gasket. The electronic components in the housing are thus protected from electromagnetic waves.
However, neither the metallic gasket, the elastomer gasket nor the meshed gasket sufficiently shields the electronic components from electromagnetic waves, and each has problems.
Often the housing the electronic components must be airtight or watertight depending on the environment in which the electronic components operate. For example, the place where a washing machine is operated is very humid. With automobiles, various environments are possible. Thus airtightness and watertightness are required for electronic-component housings for washing machines and automobiles. The electromagnetic-shielding gasket must have conductivity or low electric resistivity to shield the electronic components from electromagnetic noise. The electromagnetic-shielding gasket must also be elastic so as to fit any gap in the housing and make the housing airtight or watertight. However, the related-art metallic gasket cannot resiliently deform or completely seal the housing. When the related-art meshed gasket is compressed, the meshed metal permanently strains, thereby deteriorating the elasticity of the elastomer.
On the other hand, the related-art elastomer gasket has high conductivity because the gasket contains conductive metallic particles or carbon black (referred to as "conductive particles"). The gasket also has elasticity because its base material is elastomer. When the content of the conductive particles is increased, the electric resistivity is reduced. However, when the content of the conductive particles exceeds a fixed amount, the elasticity of the elastomer deteriorates. Consequently, the content of the conductive particles has an upper limit, so that the elasticity of the elastomer can be maintained. However, even at that upper limit, the electric resistivity of the elastomer gasket exceeds 10 ohm.cm, and the elastomer gasket provides an insufficient electromagnetic-shielding effect.